1. Field of the Invention
The present invention relates to a code generator for text transmission devices which, in order to permit a subscriber to be recognized, emits the code characters assigned to the subscriber over a trunk line, wherein the code characters are stored in a memory which is formed from electronically operable storage elements whose information content remains unchanged upon disconnection of the operating voltage, and more particularly to a code generator in which a write-in control unit is provided by means of which the code characters are written into the memory, and wherein a read-out control unit is provided by means of which the code characters are read from the memory.
2. Description of the Prior Art
Text transmission devices are normally provided with code generators. The code generator of a first subscriber is interrogated following the establishment of a connection between the first subscriber and a second subscriber. The code generator emits a sequence of code characters which is assigned to the first subscriber and with the aid of which the second subscriber ca determine whether a correct connection has been established. It is also possible for the code generator to be triggered by the relevant subscriber.
It is already generally well known in the art to employ mechanical code generators in teleprinters. With this type of code generator, the characters are stored in combs arranged radially on a roller. The number of combs is equal to the number of characters to be emitted by the code generator. Each comb is provided with teeth, the number of which corresponds to the number of elements of a code character. Each tooth is assigned to one information element of the code character with start element polarity, and for each information element of the code character with stop element polarity one tooth must be broken from the comb. The characters are interrogated in that the roller is moved and contacts are opened or closed depending upon the presence or absence of the teeth. This known code generator has the disadvantage that a change of the characters necessitates mechanical intervention. Furthermore, due to the need to employ a transport magnet, it has a large power consumption. The contacts require interference suppression elements in order to ensure satisfactory cooperation with the electrical units of the teleprinter. In order to return the code generator to its basic position, elaborate procedures are required if, for example, during the transmission of the characters, the connection to the second subscriber is broken or the main power supply fails. Furthermore, the known code generator has the disadvantage that it contains moving mechanical components which are subject to wear and which give rise to noise.
U.S. Pat. No. 3,968,330, issued to Martin Ludwig on July 6, 1976 and assigned to the same assignee as the present invention, discloses a code generator which contains a memory which is constructed from electronically operable storage elements and which stores the code characters, and wherein the characters are not changed following the disconnection of the operating voltage. Memories of this kind are also referred to as electronically alterable fixed word memories. In this known code generator, the code characters are written into the memory wth the aid of a write-in control unit arranged on a plug-in assembly. The write-in control unit is plugged in only when required during installation, in order to input the code characters, for example by a service technician. The code characters are input with the aid of a keyboard or a punched tape read-out device which forms a part of the relevant device. If the memory consists of an unalterable fixed word memory, write-in into the memory is effected, for example, by the manufacturer and to enable it to be exchange, if required, it is arranged as a plug-in device.
If the write-in into the code generator is effected on the device itself, or the memory is exchangeable, the danger of adulteration by the user exists, if the user should come into possession of the write-in control unit or a modified memory module. Furthermore, this code generator has the disadvantage that the construction of the devices requires that particularly skilled personnel be entrusted with the modification of the code characters.